Because 6.0 mg/L is greater than 1.44 mg/L, enough dissolved oxygen is present in the water.

12.9.4 Chemical Requirements in Ferric Coagulation Treatment

The chemical reactions for the ferric coagulants FeCl3 and Fe2(SO4)2 involve precipitations in the form of ferric hydroxide. Calcium bicarbonate is always present in natural waters, so it must first be satisfied before any external source of the hydroxide ion is provided. This hydroxide can be provided using lime, and as in the case of alum, this is the hydroxide that will be utilized in the coagulation reactions to be discussed. The respective reactions for the satisfaction of calcium bicarbonate are as follows:

2FeCl3 + 3Ca(HCO3)2 ^ 2Fe(OH)3i + 6CO2 + 3CaCl2 (12.67)

Fe2(SO4)3 + 3Ca(HCO3)2 ^ 2Fe(OH)3i + 6CO2 + 3CaSO4 (12.68)

The precipitation of Fe(OH)3 is optimal at a pH of 8.2, therefore, the coagulation pH should be adjusted to this value. This is assuming the coagulation temperature is 25°C and at a solids concentration of 140 mg/L.

For the two reactions, the number of reference species is 6. Thus, the equivalent masses are ferric chloride = 2FeCl3/6 = 54.1, ferric sulfate = Fe2(SO4)3/6 = 66.65 and calcium bicarbonate = 3Ca(HCO3)2/6 = 81.05.

M CaOakgeqFeII

Upon satisfaction of the natural alkalinity, other alkalinity sources may be used such as lime, caustic soda, and soda ash. Also, as in the case of alum coagulation, alkalinity requirements are usually expressed in terms of CaCO3. Therefore, we also express the reactions of the ferric salts in terms of calcium carbonate. The respective chemical reactions are:

Note: In order to find the equivalent masses, the same number of atoms of iron in the balanced chemical reactions as used in Eqs. (12.67) and (12.68) should be used in Eqs. (12.69) through (12.70).

Otherwise, the equivalent masses obtained will not be equivalent to each other. From the reactions, the equivalent mass of lime = 3CaO/6 = 28.05.

As determined by a jar test, let [FeIIIopt]mg and [FeIIIopt]geq be the milligrams per liter and gram equivalents per liter of optimum ferric salt dose, respectively, and let V be the cubic meters of water or wastewater treated. Also, let MCaOkgeqPeIII and MCaOFeIII be the kilogram equivalents and kilogram mass of lime, respectively, used at a fractional purity of PCaO. In the case of Ca(HCO3)2, the respective symbols are MCa(HCO3)2kgeqFeIII and MCa(HCO3)2FeIII at a fractional purity of PCa(HCO3)2. PCa(HCO3)2

is, of course, equal to 1 in natural waters.

The number of kilogram equivalents, MFeIIIkgeq, of any one of the ferric salts needed is ([FeIIIopt]geq(1000)/1000)V = [FeIIIopt]geqV. For FeCl3, [FeIIIopt]geqV = [FeIIIopt]mgV/1000(54.1) and that for Fe2(SO4)3, it is [FeIIIopt]geqV = [FeIIIopt]mg V/1000(66.65). Let MFeIIIClkeq and MFeIIICl be the kilogram equivalents and kilograms of FeCl3 used, respectively, at a fractional purity of PFeIIICl. Also, let Mmh^ keq and M FeIIISO be the kilogram equivalents and kilograms of Fe2(SO4)3 used, respectively, at a fractional purity of PFeIIIS^. Thus,

As in the case of alum, the most general notion is that MFeIIIkgeq is reacted by all the alkalinities. Let /ran&^œ^ be the fraction of MFeIIIkgeq reacted by calcium bicarbonate and /jeIIICaO, the fraction reacted by lime. If calcium bicarbonate and lime are the only alkalinities reacting with the ferric salt, /ranoi^œ,^ + /ranao = 1. We now have

Example 12.6 A raw water containing 140 mg/L of dissolved solids is subjected to a coagulation treatment using Fe2(S04)3. If the optimum coagulant dose as determined by a jar test is 40 mg/L, calculate the alkalinity of calcium bicarbonate required. If the natural alkalinity is 100 mg/L as CaC03, is there enough alkalinity to neutralize the coagulant dose?

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